Fabrication of tin-based halide perovskites by pulsed laser deposition
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S.I. : CURRENT STATE-OF-THE-ART IN LASER ABLATION
Fabrication of tin‑based halide perovskites by pulsed laser deposition Sarah Hoffmann‑Urlaub1 · Yaodong Zhang1 · Zhaodong Wang1 · Birte Kressdorf1 · Tobias Meyer2 Received: 2 December 2019 / Accepted: 3 June 2020 © The Author(s) 2020
Abstract Mixed-organic-cation perovskite absorbers as formamidinium doped methylammonium tin iodine (NH2 CH)1−x (CH3 NH3 )x SnI3 ( x ≤ 1 ) can provide a pathway to highly efficient lead-free solar cells. Although this class of materials is known to be severely susceptible to degradation, induced among others by enhanced temperatures, humidity and illumination, an improved layer quality in view of crystal size and homogeneity is the key to diminish or even to block certain degradation channels. In this work, we present the fabrication of fully tin-based perovskites via pulsed laser deposition. The morphology is analyzed for different deposition energies and temperatures to find the optimum process window. The thin films already reveal crystalline structure at room temperature, while they are smooth and homogeneous above a critical thickness for carefully adapted deposition parameters. In contrast to the assumption that at elevated temperatures, the crystallinity is improved, and we find that the films reveal a strong organic depletion and simultaneously tin enrichment. As a measure for their suitability to be employed as photovoltaic absorbers, the band gap of the differently doped perovskites is estimated by spectroscopic ellipsometry in the range of 1.3 to 1.4 eV. Keywords Lead-free organic-inorganic halides · Pulsed laser deposition · Mixed-cation tin perovskites · Ellipsometry · Solar cells
1 Introduction Thin film hybrid organic–inorganic perovskite (HOIP) solar cells were identified to be a suitable alternative to conventional Si-cells, since they are cheap, scalable and fabricated from abundant materials. In terms of light-harvesting properties, they are capable to even exceed the latter in view of power conversion efficiency (PCE) due to their higher theoretical Shockley–Queisser limit of 30.5% [11]. The most important component is the absorber layer that consists of a metal halide in perovskite structure, where metals from group IV (Ge, Sn and Pb) and organic complexes as methylammonium iodine (MAI) and formamidinium iodine (FAI) are frequently used. These complexes reveal exceptional photovoltaic properties due to a balanced transport of * Sarah Hoffmann‑Urlaub [email protected] 1
Institute of Materials Physics, University of Göttingen, Friedrich‑Hund‑Platz 1, 37077 Göttingen, Germany
4th Institute of Physics ‑ Solids and Nanostructures, University of Göttingen, Friedrich‑Hund‑Platz 1, 37077 Göttingen, Germany
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electrons and holes, high absorption coefficients [44], direct and tunable band gaps [36] and long carrier diffusion lengths [32, 45]. Hence, a lot of effort was put in developing perovskite solar cells, resulting in an increase of PCE from 3.8 to 25.2% [52] within a few years. However, record breaking ce
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